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Photoelectron spectroscopy and low-energy electron diffraction (LEED) have been used to examine the electronic structure and crystallinity of thin films of ${\mathit{A}}_{\mathit{X}}$${\mathrm{C}}_{60}$ where A=Na, K, Rb, and Cs and 0\ensuremath{\le}x=6. For undoped ${\mathrm{C}}_{60}$ films, temperature-dependent LEED studies show changes that correspond to the lattice transformation from the simple-cubic to the face-centered-cubic structure. For doped ${\mathrm{C}}_{60}$ films, the LEED results show a decrease in the quality of the LEED pattern upon the nucleation of the body-centered A-${\mathrm{C}}_{60}$ phases. Spectroscopic studies of these fullerides indicate that the effects of electron correlation are always important. In particular, the ${\mathit{A}}_{1}$${\mathrm{C}}_{60}$ phases of Rb and Cs are characterized by an occupied-valence-band feature \ensuremath{\sim}0.5 eV wide centered \ensuremath{\sim}0.25 eV below ${\mathit{E}}_{\mathit{F}}$ that is derived from the lowest unoccupied molecular orbitals (LUMO) of ${\mathrm{C}}_{60}$. The much greater width relative to band calculations is attributed to electron correlation. For these phases, there is also emission at the Fermi level, despite the fact that transport studies indicate insulating character. This implies that the electronic states at ${\mathit{E}}_{\mathit{F}}$ are localized. The ${\mathit{A}}_{3}$${\mathrm{C}}_{60}$ phase of K and Rb exhibit a metallic Fermi-level cutoff. Spectroscopic features 0.3 and 0.7 eV below ${\mathit{E}}_{\mathit{F}}$ are also observed that are not reproduced in one-electron band-structure calculations. The ${\mathit{A}}_{4}$${\mathrm{C}}_{60}$ phases of K, Rb, and Cs all exhibit insulating character with a split LUMO band. All of the ${\mathit{A}}_{6}$${\mathrm{C}}_{60}$ phases are insulators with a filled LUMO band. For Na-${\mathrm{C}}_{60}$, the valence-band spectra show no emission at ${\mathit{E}}_{\mathit{F}}$ for any Na concentration. Finally, photoemission results showed partial occupation of the (LUMO+1)-derived levels, corresponding to ${\mathrm{C}}_{60}^{8\mathrm{\ensuremath{-}}}$ for isolated ${\mathrm{C}}_{60}$ molecules deposited onto multilayers of Na, K, and Rb at 40 K.

The electrical properties of the alkali-metal fullerides have been investigated via parallel resistivity measurements and photoelectron spectroscopy. Phase formation associated with the incorporation of Na, K, Rb, and Cs into ${\mathrm{C}}_{60}$ films is reflected by the dependence of resistivity on temperature and alkali-metal concentration. Photoemission spectra show the details of the filling of states derived from the lowest unoccupied molecular level and the consequences of electron correlation. For K-${\mathrm{C}}_{60}$ and Rb-${\mathrm{C}}_{60}$ films, the resistivity and photoemission results indicate metallic character only for the ${\mathit{A}}_{3}$${\mathrm{C}}_{60}$ phase. Changes in the temperature coefficient of resistivity for Rb-${\mathrm{C}}_{60}$ as a function of doping reveal the granular nature of the film and show the transition from insulating to metallic character. In contrast, the Na and Cs fullerides are insulators for all concentrations. The temperature-dependent resistivity results for ${\mathrm{Na}}_{\mathit{x}}$${\mathrm{C}}_{60}$ show a phase transformation at 226 K for x between two and three, but neither phase is metallic.

High-resolution photoemission provides the signatures of ${\mathrm{K}}_{3}$${\mathrm{C}}_{60}$(111), ${\mathrm{K}}_{4}$${\mathrm{C}}_{60}$, ${\mathrm{K}}_{6}$${\mathrm{C}}_{60}$, and the dilute solid solution of K in ${\mathrm{C}}_{60}$(111), showing the effects of electron-electron correlation in all of the K-${\mathrm{C}}_{60}$ phases. ${\mathrm{K}}_{3}$${\mathrm{C}}_{60}$(111) is a metal with a distinct Fermi-level cutoff but with spectral broadening near ${\mathit{E}}_{\mathit{F}}$ not predicted by band calculations. ${\mathrm{K}}_{4}$${\mathrm{C}}_{60}$ is an insulator, although band calculations predict metallic character. Photoemission and low-energy-electron-diffraction results demonstrate kinetically limited growth for K-${\mathrm{C}}_{60}$ films produced by vapor deposition.

Core-level photoemission studies of alkali fulleride ${\mathit{A}}_{\mathit{x}}$${\mathrm{C}}_{60}$ thin films indicate alkali bonding configurations that can be associated with octahedral and tetrahedral interstitial sites of the ${\mathrm{C}}_{60}$ lattice. For ${\mathrm{K}}_{\mathit{x}}$${\mathrm{C}}_{60}$, the K 2p core-level results suggest ${\mathrm{K}}_{3}$${\mathrm{C}}_{60}$ nucleation for K concentration as low as x\ensuremath{\sim}0.1 with occupation of both octahedral and tetrahedral sites. In contrast, the Rb 3d and Cs 4d core-level spectra show filling of only octahedral sites until x\ensuremath{\sim}1, forming ${\mathit{A}}_{1}$${\mathrm{C}}_{60}$ phases. For intercalation beyond x\ensuremath{\sim}1, the fcc tetrahedral sites of Rb are occupied as ${\mathrm{Rb}}_{3}$${\mathrm{C}}_{60}$ forms. Intercalation with K or Rb beyond x\ensuremath{\sim}3, or with Cs beyond x\ensuremath{\sim}1, results in a spectral feature that is consistent with formation of a body-centered phase with equivalent tetrahedral interstitial sites. Results for ${\mathrm{Na}}_{\mathit{x}}$${\mathrm{C}}_{60}$ suggest that Na ions occupy off-center positions in the large octahedral sites. Studies of ternary fullerides ${\mathrm{K}}_{\mathit{x}}$${\mathit{A}}_{\mathit{y}}$${\mathrm{C}}_{60}$ (x+y3) demonstrate preferential filling of the octahedral sites by the larger ion.

Photoemission and inverse photoemission studies of thin films of ${\mathrm{C}}_{60}$ and ${\mathit{C}}_{70}$ reveal the distribution of occupied and empty electronic states of these molecular solids. X-ray photoemission results also show the C 1s main line and features related to \ensuremath{\pi}-${\mathrm{\ensuremath{\pi}}}^{\mathrm{*}}$ shakeups, electron energy losses, and plasmons. Potassium doping produces changes that can be related to the occupation of states derived from the lowest unoccupied molecular orbitals of the fullerenes and band-structure effects. Important differences are observed upon K doping of ${\mathrm{C}}_{60}$ and ${\mathrm{C}}_{70}$, particularly in states near the Fermi level, and these would be reflected in the electron-phonon coupling, superconductivity, and the phase diagram. Resistivity measurements for ${\mathrm{K}}_{\mathit{x}}$${\mathrm{C}}_{60}$ show a resistivity minimum for ${\mathrm{K}}_{3}$${\mathrm{C}}_{60}$ and a dependence on stoichiometry that is indicative of dispersed conducting micrograins in an insulating medium. Oxygen-exposure studies demonstrate that ${\mathrm{K}}_{\mathit{x}}$${\mathrm{C}}_{60}$ thin films are unstable.

Photoemission studies of Li and Na fullerides show a band of alkali-metal-induced states that is fully below the Fermi level for a stoichiometry of ${\mathit{A}}_{2}$${\mathrm{C}}_{60}$ and inverse photoemission results show a splitting of the unoccupied bands. Equivalent results for the Rb fullerides show the formation of only a metallic phase, ${\mathrm{Rb}}_{3}$${\mathrm{C}}_{60}$. For all three fullerides, doping to saturation produces nonmetallic ${\mathit{A}}_{6}$${\mathrm{C}}_{60}$ phases.

Studies of photoinduced oxidation of physisorbed ${\mathrm{O}}_{2}$ on GaAs(110) at 25 K show that the surface reaction rate is strongly dependent on photon energy and substrate doping type. With 1.7-eV photon irradiation, the reaction cross section is 6.1\ifmmode\times\else\texttimes\fi{}${10}^{\mathrm{\ensuremath{-}}20}$ ${\mathrm{cm}}^{2}$ for n-type GaAs and \ensuremath{\sim}7 times higher for p-type GaAs. With 1.97-eV photon irradiation, the reaction cross section is 1.5\ifmmode\times\else\texttimes\fi{}${10}^{\mathrm{\ensuremath{-}}18}$ ${\mathrm{cm}}^{2}$ for n-type GaAs and \ensuremath{\sim}8 times higher for p-type GaAs. We show that the energy distribution of the photoexcited electrons in the conduction band relative to the ${\mathrm{O}}_{2}$ electron-affinity level is the critical parameter in determining reaction with the substrate. Differences in the reaction rate reflect the energy dependence of the resonance coupling of excited substrate electrons to physisorbed ${\mathrm{O}}_{2}$. Surface band bending and the photon excitation energy control the matching of these electrons with the ${\mathrm{O}}_{2}$ electron-affinity level.